Obstructive sleep apnea syndrome (OSAS) is a well recognized disorder which may affect as much as 1-5% of the adult population. It is one of the most common causes of excessive daytime somnolence, and it is the single most frequent reason for referral to sleep disorder clinics.
The syndrome is characterized by the intermittent obstruction of the upper airway which occurs during sleep. The obstruction results in a spectrum of respiratory disturbances ranging from the total absence of airflow (apnea) to significant obstruction with or without reduced airflow (hypopnea and snoring), despite continued respiratory efforts. The morbidity of the syndrome arises from hypoxemia, hypercapnia, bradycardia and sleep disruption associated with the apneas and arousals from sleep. OSAS is most frequent in obses males, and is associated with all conditions in which there is anatomic or functional narrowing of the upper airway, as in heavy snoring.
The pathophysiology of OSAS is not fully worked out. However, it is now well recognized that obstruction of the upper airway during sleep is in part due to the collapsible behavior of the supraglottic segment during the negative intraluminal pressure generated by inspiratory effort. Thus, the human upper airway during sleep behaves as a Starling resistor, which is defined by the property that the flow is limited to a fixed value irrespective of the driving (inspiratory) pressure. Partial or complete airway collapse can then occur associated with the loss of airway tone which is characteristic of the onset of sleep and may be exaggerated in OSAS.
Since 1981, continuous positive airway pressure applied by a tight fitting nasal mask worn during sleep has evolved as the most effective treatment for this disorder, and is now the standard of care. The availability of this non-invasive form of therapy has resulted in extensive publicity for apnea and the appearance of large numbers of patients who previously may have avoided the medical establishment because of the fear of tracheostomy. Increasing the comfort of the system, which is partially determined by minimizing the necessary nasal pressure, has been a major goal of research aimed at improving patient compliance with therapy. Various systems for the treatment of obstructive sleep apnea are disclosed, for example, in "Reversal of Obstructive Sleep Apnea by Continuous Positive Airway Pressure Applied Through The Nares", Sullivan et al, Lancet, 1981, 1:862-865; and "Reversal Of The `Pickwickian Syndrome` By Long-Term Use of Nocturnal Nasal-Airway Pressure"; Rapoport et al, New England Journal of Medicine, Oct. 7, 1982.
The article "Induction of upper airway occlusion in sleeping individuals with subatmospheric nasal pressure", Schwartz et al, Journal of Applied Physiology, 1988, 64, pp 535-542, also discusses various polysomnographic techniques.
Despite its success, limitations to the use of nasal CPAP exist. These mostly take the form of discomfort from the mask and the nasal pressure required to obliterate the apneas. Systems for minimizing the discomfort from the mask are disclosed, for example, in U.S. Pat. Nos. 4,655,213, Rapport et al, and 5,065,756, Rapoport, as well as in "Therapeutic Options For Obstructive Sleep Apnea", Garay, Respiratory Management, July/August, 1987, pp 11-15; and "Techniques For Administering Nasal CPAP", Rapoport, Respiratory Management, July/August 1987, pp 18-21. Minimizing the necessary pressure remains a goal of the preliminary testing of a patient in the sleep laboratory. However, it has been shown that this pressure varies throughout the night with sleep stage and body position. Furthermore, the therapeutic pressure may both rise or fall with time in patients with changing anatomy (Nasal congestion/polyps), change in weight, changing medication or with alcohol use. Because of this, most sleep laboratories currently prescribe the setting for home use of nasal CPAP pressure based upon the single highest value of pressures needed to obliterate apneas during a night of monitoring in the sleep laboratory. Retesting is often necessary if the patient complains of incomplete resolution of daytime sleepiness, and may reveal a change in the required pressure.